Common obesity and the metabolic syndrome may therefore result from an anomalous adaptive winter response. The stimulus for the winter response is proposed to be a fall in vitamin D. The synthesis of vitamin D is dependent upon the absorption of radiation in the ultraviolet-B range of sunlight. At ground level at mid-latitudes, UV-B radiation falls in the autumn and becomes negligible in winter. It has previously been proposed that vitamin D evolved in primitive organisms as a UV-B sensitive photoreceptor with the function of signaling changes in sunlight intensity. It is here proposed that a fall in vitamin D in the form of circulating calcidiol is the stimulus for the winter response, which consists of an accumulation of fat mass (obesity) and the induction of a winter metabolism (the metabolic syndrome). Vitamin D deficiency can account for the secular trends in the prevalence of obesity and for individual differences in its onset and severity. It may be possible to reverse the increasing prevalence of obesity by improving vitamin D status.